An electric arc furnace generally comprises a melting vessel made up of a refractory hearth and surrounding sidewalls, a removable roof that closes the vessel, the vessel being associated with one or more electrodes connected to a current source. Each electrode is fitted to the end of a supporting arm overhanging the vessel and penetrates vertically into the vessel through a special hole made for this purpose in the removable roof. The furnace can be supplied either from an alternating current source using three electrodes, or from a direct current source using one or more electrodes. In this case, one or more electrodes are used, each passing through the vault of the furnace and each associated with one or more electrodes mounted in the hearth and each connected to the current source by a return conductor. The furnace is of course associated with a certain number of other items of equipment such as the electrical installations, scrap and additive charging devices, devices for evacuating the molten steel and slag and fume evacuating circuits.
It is further useful in order to improve the efficiency of the furnace to provide a means of preheating the scrap charge before loading it into the furnace. A popular preheating method for doing this consists in passing hot gasses through a preheating enclosure.
This fairly complex assembly of costly, bulky apparatus is installed in a building that is normally arranged in three zones: a scrap metal reception and recovery zone, a preheating zone and a metal-making zone.
The raw scrap is brought to the reception zone in one or more transport containers or "buckets". The buckets are picked up and carried between zones by an overhead travelling crane running the whole length of the installation.
The bottom of each bucket is generally made up of two shells, each pivoted about a horizontal axis, and which, by moving away from each other, let the charge fall into the vessel, the roof of which having been opened. The roof vault and electrodes can be suspended from a gantry that can move horizontally or, preferably, suspended from arms that swing horizontally about a vertical axis on a base next to the furnace.
Because steel production generates a large volume of hot and dusty gasses during the heating and metal-making stages, the resulting fumes are collected by a hood connected to an outwardly fume evacuation circuit via a dedusting means.
The hot gases serving to preheat the scrap can be produced from these fumes in a combustion chamber which serves to burn off carbon monoxide and other unburnt residue and also to recover a part of the dust carried by the fumes. The burnt gases are directed to one or more preheating cells containing scrap waiting to be loaded into the vessel of the furnace.
The travelling crane picks up a bucket from the reception zone, each bucket containing a load of cold scrap, and carries it to the preheating cell where it is left for the necessary period of time. After preheating, the crane picks up the bucket and positions it over the vessel, the scrap then being dropped into the vessel by opening the bottom of the bucket.
During the charging operation, the roof is open, and a large quantity of fumes escape uncollected. Steel and slag may also be projected out of the furnace as the scrap falls into the vessel.
Steel producing installations are also very noisy, and so one design objective is to provide as much insulation as possible in order to reduce fume and noise pollution.
To this end, it has been suggested to combine the preheating zone and the metal-making zone in a closed chamber within which the majority of the fumes and dust produced can be collected and sucked out by a hood connected to a dedusting circuit. An installation of this kind is described, for example, in U.S. Pat. No. 4,506,370. In this installation, scrap is preheated in a chamber next to the furnace, the whole assembly being placed inside a chamber of a relatively reduced size. The preheating chamber is limited by a tubular side wall having a top and bottom that can be closed in a movable fashion. The hot fumes from the furnace are collected by a duct opening into the upper portion of the chamber in which burners are fitted, thus constituting the combustion chamber. The burnt gases flow through the chamber and leave through a lower opening connected to a duct that vents to atmosphere.
More importantly, the preheating chamber is further designed as a container with an opening for charging the furnace with scrap. The chamber is mounted on a support carriage or travelling crane which carries the chamber between the preheating position next to the furnace and the unloading position directly above the furnace where the scrap is emptied directly into the vessel by releasing the opening bottom.
The chamber is loaded with a new charge of scrap outside the closed chamber by a conventional loading bucket.
The support carriage that carries the preheating chamber moves it accordingly to the following positions and in the following order: 1) the cold scrap loading position outside the closed chamber; 2) the preheating position next to the furnace, the chamber being coupled to hot gas input and output ducts; 3) the unloading position above the furnace.
The fume collecting chamber must be fitted with a first gate so that the preheating chamber can move between the cold scrap loading position outside the chamber and the preheating position.
After the melting stage, the steel is poured into ladles which must themselves be moved in and out through another opening fitted with a second gate.
The large size of these gates represents a leak risk, and that maneuvering the gates hinders the operation of the furnace.
Moreover, the chamber transfer carriage is relatively bulky and has to travel an appreciable distance between the cold scrap loading position outside the furnace and the scrap unloading position above the furnace. Needless to say, the path travelled by the transfer carriage and chamber must be clear, and, more importantly, the gas circuits must be set out in such a way as to ensure easy connection and disconnection and likewise stay clear of the path of the transfer carriage.
To overcome these drawbacks, EP-0 225 939 suggests that the preheating and loading chamber be stationary and the furnace movable. The dimensions of the fume evacuation chamber can be significantly reduced, the chamber reduced in fact to simply an upper section surrounding the base of the preheating chamber designed to allow the bottom to open, and a lower section occupied by the furnace, the furnace being mounted on a transfer carriage that moves between the unloading position in which the furnace is positioned beneath the preheating chamber, and the metal-making position outside the fume collection chamber. In this position the furnace is closed off by the roof carrying the electrodes and comprising a hole that connects to a fume evacuation pipe leading to the combustion chamber installed in the top part of the stationary preheating chamber. The advantages of such an arrangement are that the dimensions of the fume collecting chamber can be reduced and that the chamber only requires a single gate for the passage of the furnace. Moreover, except for the removable connection on the furnace roof, all the hot gas circuits are stationary and can be arranged around the preheating chamber.
The furnace, however, must be mounted on a movable carriage and fixed to the cradle in a removable manner which allows the furnace to be tilted in the usual way to pour the molten metal. Such an arrangement, however, complicates the installation, given the size and weight of the furnace. In addition, since the preheating chamber is stationary, cold scrap is loaded in a zone containing the various hot gas circulation circuits and could therefore be damaged, as could the combustion chamber in the top part of the chamber.